Accumulated other comprehensive loss - Summary of amounts reclassified from accumulated other comprehensive loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income tax expense (benefit)	$ (11.6)	$ (18.5)	$ (11.3)	$ 4.8
Interest expense	30.1	39.0	59.2	73.8
Net income (loss)	1.8	16.3	57.7	(47.6)
Prior service cost | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other expense, net	0.1	0.1	0.1	0.1
Amortization of defined benefit pension items: | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income tax expense (benefit)	0.0	0.0	0.0	0.0
Net of tax	0.1	0.1	0.1	0.1
Cash flow hedges: | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income tax expense (benefit)	(2.5)	0.9	(0.3)	1.9
Net of tax	5.8	(2.8)	0.8	(5.6)
Net income (loss)	5.9	(2.7)	0.9	(5.5)
Cash flow hedges: | Reclassification out of Accumulated Other Comprehensive Income | Interest rate swap contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Interest expense	2.2	(3.7)	3.1	(7.5)
Cash flow hedges: | Reclassification out of Accumulated Other Comprehensive Income | Cross-currency swap contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Interest expense	$ 6.1	$ 0.0	$ (2.0)	$ 0.0